doc: mention gccgo status in 1.9 release notes

Updates #20587

Change-Id: If35868345d9697d4afa2e9c19b85e01d94c87908
Reviewed-on: https://go-review.googlesource.com/47034Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.9.html

@@ -190,6 +190,16 @@ type T1 = T2
   <a href="/pkg/net/http/#ProxyFromEnvironment"><code>http.ProxyFromEnvironment</code></a>.
 </p>
 
+<h3 id="gccgo">Gccgo</h3>
+
+<p>
+Due to the alignment of Go's semiannual release schedule with GCC's
+annual release schedule,
+GCC release 7 contains the Go 1.8.3 version of gccgo.
+We expect that the next release, GCC 8, will contain the Go 1.10
+version of gccgo.
+</p>
+
 <h3 id="tools-TODO">TODO</h3>
 
 <p>TODO: finish documenting misc tool changes</p>